Plant Operator Selection System Test Process Control and Instrumentation 1 — Questions and Answers
Question 1: A thermocouple measures temperature by:
- Expanding a bimetallic strip proportional to heat
- Generating a small voltage at the junction of two dissimilar metals (Correct answer)
- Changing resistance as temperature increases
- Detecting infrared radiation emitted by the process
Correct answer: Generating a small voltage at the junction of two dissimilar metals
Thermocouples produce a millivolt signal (Seebeck effect) at the junction of two different metals that varies predictably with temperature.
Question 2: In a PID controller, the 'I' (integral) term is used to:
- Respond to the current error value
- Eliminate steady-state offset by accumulating past error over time (Correct answer)
- Dampen rapid changes by anticipating future error
- Set the maximum allowable output
Correct answer: Eliminate steady-state offset by accumulating past error over time
The integral term sums past errors over time and applies a correction that eliminates the steady-state offset left by proportional control alone.
Question 3: A control valve with an air-to-open (ATO) fail action will:
- Open fully when instrument air pressure is lost
- Close fully when instrument air pressure is lost (Correct answer)
- Remain at its last position when air pressure is lost
- Open slowly over 30 seconds after air loss
Correct answer: Close fully when instrument air pressure is lost
Air-to-open (also called fail-closed) valves close when air supply is lost, which is the safe failure mode for many process streams.
Question 4: What does a differential pressure (DP) transmitter measure when installed across an orifice plate?
- Temperature of the flowing fluid
- Flow rate based on the pressure drop across the restriction (Correct answer)
- The absolute pressure of the upstream pipe
- Fluid density at operating conditions
Correct answer: Flow rate based on the pressure drop across the restriction
Flow through an orifice creates a pressure drop proportional to the square of the flow rate, which the DP transmitter converts to a flow signal.
Question 5: On a Process and Instrumentation Diagram (P&ID), a dashed line connecting an instrument to a control element typically represents:
- A mechanical connection
- An electrical or pneumatic signal (Correct answer)
- A proposed future connection
- A safety interlock bypass
Correct answer: An electrical or pneumatic signal
Dashed lines on P&IDs represent signal lines (electrical, pneumatic, or digital) that transmit information between instruments and controllers.
Question 6: A level transmitter in a tank reads higher than the actual level. The most likely cause is:
- The tank is overfilled
- A plugged or blocked impulse line on the low-pressure side (Correct answer)
- The control valve is stuck open
- The temperature sensor is reading low
Correct answer: A plugged or blocked impulse line on the low-pressure side
A blocked low-pressure impulse line traps pressure, reducing the differential and making the instrument report a falsely high level.
Plant Operator Selection System Test Process Control and Instrumentation 1
A thermocouple measures temperature by: